Hybrid drives for motor vehicles have been known for a long time. In U.S. Pat. No. 3,566,717, the entire contents of which is incorporated herein by reference, a drive system for motor vehicles is disclosed in which the shaft of the internal combustion engine is coupled to the first shaft of a three-shaft epicyclic gearing. A first electric motor is coupled to the second shaft of the three-shaft epicyclic gearing. The third shaft is coupled to a second electric motor and to the wheels of the vehicle. The power transfer is carried out as power split, partly on the mechanical path via the third shaft of the epicyclic gearing and the remaining part on the electrical path from the first to the second electric motor. The transmission ratio, which acts from the internal combustion engine on the wheels of the vehicle is set by the speed ratio of the internal combustion engine and the first electric motor. The known arrangement may be used as a stepless gear mechanism but due to the internal power flows is only suitable for one direction of travel.
A drive system for motor vehicles has become known from DE 101 41 923, the entire contents of which is incorporated herein by reference, in which the shaft of the internal combustion engine is coupled to the first shaft of a three-shaft epicyclic gearing. A first electric motor is connected to the second shaft of the epicyclic gearing and the third shaft is coupled to a second electric motor and the wheels of the vehicle. For reverse driving, the connection of the third shaft of the epicyclic gearing to the second electric motor and the wheels of the vehicle is separated by a clutch, and the shaft of the internal combustion engine and the shaft of the first electric motor are connected by means of a clutch, so that electrical power transfer is possible in series from the first electric motor operating in the manner of a generator to the second electric motor. For switching between power split mode and series mode it is essential to reduce the power transferred from the internal combustion engine to the wheels to zero.
A drive system for industrial trucks has become known from DE 103 40 472, the entire contents of which is incorporated herein by reference, in which the shaft of the internal combustion engine is coupled to the shaft of a three-shaft epicyclic gearing. A first electric motor is connected to the second shaft of the epicyclic gearing, and a third shaft is connected to a change gear, the output shaft thereof being connected to a summing gear. A second electric motor is connected to the other input shaft of the summing gear, the output thereof acting on the wheels. A lock-up clutch on the three-shaft epicyclic gearing allows electrical operation in series, in which power generated by the internal combustion engine is converted by the first electric motor into electrical current, which is supplied to the second electric motor as drive current. To this end, the change gear is shifted into a neutral position. Forward and reverse driving is possible in series mode. Moreover, forward and reverse driving is possible in power split mode, by the change gear being shifted into the shift stage belonging to the respective direction of travel. For shifting between power split mode and series mode, when using a transmission step associated with the lock-up clutch, it is possible to keep constant the power transferred from the internal combustion engine to the wheels in a speed synchronous power shift.
Generally, the clutches are designed as hydraulically actuated multi-plate clutches. In the known manner hydraulically actuated clutches generally comprise co-rotating pistons so that the pressure medium required for actuation has to be transferred to rotating parts which is structurally expensive and susceptible to maintenance.
The object of the invention is to provide a hybrid drive system for industrial trucks which offers a stepless adjustment of the transmission, good controllability and good efficiency. Operating functions of the industrial truck are also intended to be fulfilled in addition to the travel drive, zero emission operation being also intended to be possible, at least occasionally, in addition to the operation of an internal combustion engine as a primary energy source, in order to drive through warehouses or containers. Finally, with the drive system high acceleration, hill climbing ability and high driving speeds are intended to be achieved both in the forward and the reverse directions of travel.